The present invention relates generally to the technical field of hydrodynamics and, more particularly, a lift generating shape used for a trawl system component.
Published Patent Cooperation Treaty (xe2x80x9cPCTxe2x80x9d) International Patent Application, International Publication Number WO 97/13407, International Publication Date Apr. 17, 1997, entitled xe2x80x9cTrawl System Cell Design and Methodsxe2x80x9d (xe2x80x9cthe First PCT patent applicationxe2x80x9d) describes a trawl system that uses corkscrew shaped mesh bars to improve the shape and performance of the trawl system. As set forth in the First PCT patent application, FIG. 1 depicts a towing vessel 10 at a surface 11 of a body of water 12 that is towing a mid-water trawl 13 of a trawl system 9. The trawl 13 is positioned between the surface 11 and an ocean bottom 14. The trawl 13 can be connected to the towing vessel 10 in many ways, such as by a main towing line 18 connected through doors 19, towing bridles 20 and mini-bridles 21, 22. A series of weights 23 is attached to mini-bridle 22. Likewise, the shape and pattern of the trawl 13 can vary as is well known in the art. As shown, the trawl 13 has a forward section 24 that includes forward projecting wings 25 for better herding at mouth 26. The forward section 24, including wings 25, is seen to define a mesh size that is larger than that used for a mid-section 27, back-end 28, or codend 29 of the trawl 13.
FIG. 2 illustrates the wing 25 of the trawl 13 of FIG. 1 in more detail and includes a series of mesh cells 30 of quadratic cross-section that are part of panel 31 and are offset from axis of symmetry 32 of the trawl 13. The size of mesh cells 30 is determined by a distance between adjacent knots or equivalent couplers 34. Different sections of the trawl 13, and even different regions within a section, use different size mesh cells 30, which generally form a repeating pattern within that section or region of a section.
As shown in FIG. 3, the mesh cells 30 each have a longitudinal axis of symmetry 30a, and are formed by mesh bars 35a, 35b, 35c and 35d each of which includes several product strands 36, 37. The product strands 36, 37 may be twisted about a longitudinal axis of symmetry 38 in either one of two lay directions: right-hand or left-hand as viewed axially along longitudinal axis of symmetry 38 and in a receding direction established upstream of the trawl 13 thereby establishing the cork-screw shape disclosed in the First PCT patent application.
As disclosed in the First PCT patent application, the mesh bars 35 may be formed either by product strands 36, 37 as depicted in FIG. 3, or by straps which are not illustrated in any of the FIGs. Published PCT International Patent Application, International Publication Number WO 98/46070, International Publication Date Oct. 23, 1998, entitled xe2x80x9cImproved Cell Design for a Trawl System and Methodsxe2x80x9d (xe2x80x9cthe Second PCT patent applicationxe2x80x9d) further discloses that the product strands 36, 37 of the mesh bars 35 are preferably formed from a substantially incompressible material. Both the First and the Second PCT patent applications are hereby incorporated by reference.
As the towing vessel 10 draws the trawl system 9 through the body of water 12 along a straight-line course, water flows through the trawl 13 substantially parallel to an arrow 50, illustrated in FIGS. 1 and 2, which is oriented parallel to the axis of symmetry 32. However, it is to be noted that as illustrated in FIG. 3 the direction in which water flows through individual mesh cells 30 of the trawl 13 is not, in general, parallel to the axis of symmetry 30a. It should also be noted that hydrodynamically the mesh bars 35 both of conventional trawl systems 9 and of trawl systems 9 assembled in accordance with the First and Second PCT patent applications are properly characterized as xe2x80x9cbluff bodyxe2x80x9d shapes. This is to be contrasted with another class of shapes, such as airplane wings, which hydrodynamicists characterized as being xe2x80x9cstreamlinexe2x80x9d shapes.
For conventional trawl systems and trawls not assembled in accordance with the disclosure set forth in the First PCT patent application, drag forces caused by movement of the trawl system through the water entrained environment tends to draw the mesh cells 30 of the trawl 13 closer to the axis of symmetry 32 thereby closing the trawl 13. Appropriately orienting the cork-screw shape of the mesh bars 35 in accordance with the description set forth in the First PCT patent application as depicted in FIG. 3 yields a trawl system 9 in which movement of mesh bars 35 through the water entrained environment generates a force on each mesh bar 35 which has a component that is directed at a right angle from the drag force component, and away from the axis of symmetry 32. The effect of the force components generated by such movement of the individual mesh bars 35 that are oriented at a right angle from the drag force component is to make the trawl system 9, particularly the trawl 13, xe2x80x9cself-spreadingxe2x80x9d thereby expanding the trawl 13 away from the axis of symmetry 32.
As set forth above, the size of mesh cells 30 is determined by the length of the mesh bars 35 between adjacent knots or equivalent couplers 34. As indicated in FIGS. 1 and 2, the size of the mesh cells 30, and correspondingly the length of mesh bars 35, varies along the length of the trawl 13. For example, the mesh bars 35 in the forward section 24 have a length of at least 10 ft (304.8 cm). Alternatively, the mesh bars 35 in the mid-section 27 of the trawl 13 have length between 10 ft. (304.8 cm) and 0.75 ft (22.86 cm). The mesh bars 35 of the back-end 28 have a length less than 0.75 ft (22.86 cm). While manual assembly of mesh cells 30 of the forward section 24 is commercially practical, as the mesh bars 35 become ever shorter toward the codend 29 manual assembly becomes progressively more costly, and therefore less and less commercially viable. Consequently, to reduce the cost of trawls 13 the general practice is to incorporate netting woven by machines into the xe2x80x9cback-endxe2x80x9d of trawls 13 such as in the codend 29, in the back-end 28, and even perhaps in some instances in the mid-section 27.
FIG. 4 illustrates a pattern used in knitting prior art, machine-made netting 51 of a type used for the xe2x80x9cback-endxe2x80x9d of conventional trawls. The knitting process for machine-made netting 51 may be understood as progressing row-by-row, from top to bottom in the illustration of FIG. 4. Knitting of machine-made netting 51 proceeds basically at approximately a right angle to the ultimate direction of water flow past the trawl 13, indicated by the arrow 50, after the machine-made netting 51 has been incorporated into a trawl 13, and the trawl system 9 is being towed through a water entrained environment.
In knitting conventional machine-made netting 51 a number of individual spools, perhaps as many as 100, concurrently feed product strands in parallel while the net knitting machine knots or braids pairs of them together at the couplers 34 alternating back-and-forth horizontally to establish a zig-zag path for the product strands 36. Thus a vertical column of L-shaped arrows 52a in FIG. 4 indicate the zig-zag path along which a single product strand, the longitudinal axis of symmetry 38 of which turns at each coupler 34, crosses the machine-made netting 51 from top to bottom of FIG. 4. Similarly a vertical column of L-shaped arrows 52b in FIG. 4, immediately to the right of the L-shaped arrows 52a, indicate the zig-zag path along which an immediately adjacent product strand crosses the machine-made netting 51 from top to bottom. In the illustration of FIG. 4, three additional vertical columns of L-shaped arrows 52c, 52d and 52e, to the right of the columns of L-shaped arrows 52a and 52b in FIG. 4, indicate paths along which yet other product strands cross the machine-made netting 51 from top to bottom in FIG. 4. In conventional machine-made netting 51, machine tied knots or braided intersections usually provide the couplers 34 which fasten ends of mesh bars 35 together in forming the mesh cells 30.
If one attempts to produce machine-made netting 51 in the conventional way described above using spools of corkscrewed product strands 36, 37 for the mesh bars 35, one could not obtain the proper lays for the mesh bars 35 that are depicted in FIG. 3. For example, one could arrange a spool of corkscrewed product strands 36, 37 to obtain the proper lay for the mesh bar 35b in FIG. 3, but then the lay of the mesh bar 35a could not reverse direction at their common coupler 34, and therefore mesh bar 35a would have a lay opposite to that depicted in FIG. 3. Similarly, one could arrange a spool of corkscrewed product strands 36, 37 to obtain the proper lay for the mesh bar 35c, but then the lay of the mesh bar 35d would be opposite to that depicted in FIG. 3.
Such machine-made lays for corkscrew shaped mesh bars 35 would yield some of the advantages disclosed in the First PCT patent application, i.e. lower drag, less vibration and lower noise. However, such machine-made lays would not produce a self-spreading codend 29, back-end 28 or mid-section 27. Rather substantially equal strength components of force oriented perpendicular to the axis of symmetry 32 for such alternating lay, machine-made mesh bars 35 would be directed away from the axis of symmetry 32 for the mesh bars 35b and 35c, but would be directed toward the axis of symmetry 32 for the mesh bars 35a and 35d. Reversing the direction of water flow past the mesh bars 35 from that indicated by the arrow 50 in FIGS. 3 and 4 merely reverses the direction of the component of force perpendicular to the axis of symmetry 32 for the mesh bars 35. Such oppositely directed components of substantially equal strength forces perpendicular to the axis of symmetry 32 merely cancel each other so movement of the mesh bars 35 through the water entrained environment yields no net force directed away from or toward the axis of symmetry 32 for machine-made mesh cells 30 formed by either product strand or strap mesh bars 35 having cross-sectional shapes such as those disclosed in the First PCT patent application.
BRIDLES are lines that intersect the frontropes and attach to the tow lines. For a particular port or starboard tow line, a pair of bridles extend from a common connection point therewith, back to the frontropes.
CODEND is a portion of a trawl positioned at the trailing end thereof and comprises a closed sac-like terminus in which the gathered marine life including fish are trapped.
FRAME is a portion of the larger sized meshes of a net or trawl upon which is overlaid (and attached by a binding) a netting of conventional twist.
LAY is the direction in which the strands wind when viewed axially and in a receding direction.
NET is a meshed arrangement of product strands that have been woven or knotted or otherwise coupled together usually at regular intervals or at intervals that vary usually uniformly along the length of the trawl.
MESH BAR is one side of a mesh cell and is composed of synthetic or natural fibers which, in accordance with the present invention, exhibit hydrofoil-like characteristics during field operation.
MESH CELL means the sides of a mesh and includes at least three sides and associated knots or equivalent couplers oriented in space. A quadratic mesh cell has four sides with four knots or couplers, and is usually arranged to form a parallelogram (including rectangular and square), with diamond-shaped mesh (trawl mesh) being preferred. A triangular mesh cell has three sides and three knots or couplers. A hexagonal mesh cell has six sides and six knots or couplers.
PANEL is one of the sections of a trawl and is made to fit generally within and about frames shaped by riblines offset from the longitudinal axis of symmetry of the trawl.
PITCH is the amount of advance in one turn of one strand twisted about another strand (or strands) when viewed axially.
PRODUCT STRAND includes the synthetic or natural fibers or filaments used to form the construction unit of the invention which is preferably but not necessarily the product of a conventional manufacturing process, usually made of nylon, polyethylene, cotton or the like twisted in common lay direction. Such strand can be twisted, plaited, braided or laid parallel to form a sub-unit for further twisting or other use within a mesh bar in accordance with the invention.
STRAND UNIT means a group of strands used to achieve ascending or descending order where such order repeats along a mesh bar.
TRAWL is a large net generally in the shape of a truncated cone trailed through a water column or dragged along a sea bottom to gather marine life including fish.
TRAWL SYSTEM includes the trawl, net or the like in association with the towlines therefor as well as the frontropes, bridles lines, and means to keep its mouth open.
An object of the present invention is to provide economically practical self-spreading net for use in the codend 29, back-end 28, and mid-section 27 of trawls 13.
Another object of the present invention is to provide machine-made net that is self-spreading.
Another object of the present invention is to provide mesh bars 35 for a trawl 13 which produce a component of force directed in a single, pre-established direction perpendicular to a drag component of force for the mesh bars 35 if water flows past the mesh bars 35 in different directions with respect to the lay of the mesh bars 35.
Briefly, employing the present invention the machine-made portions of a self-spreading trawl 13 may be assembled using pairs of mesh bars 35 which meet at a common coupler 34, and are made from a continuous length of material having:
1. a lay with a common direction throughout the length of material;
2. a cross-sectional shape; and
3. a longitudinal axis of symmetry.
In establishing the zig-zag pattern used in knitting machine-made netting 51, the longitudinal axis of symmetry 38 of such pairs of mesh bars 35 turns at each coupler 34. During field operations, for machine-made portions included in a trawl 13 upon being towed through a water entrained environment:
1. water respectively flows past pairs of mesh bars 35 in accordance with the present invention in two different directions with respect to the common lay thereof;
2. the directions in which water flows past the product strands 36 is neither parallel nor perpendicular to the longitudinal axis of symmetry 38 of the mesh bars 35; and
3. as water flows past the mesh bars 35, the cross-sectional shapes of the mesh bars 35 produce a net component of force that is oriented in a direction perpendicular to a combined drag component of force for the mesh bars 35.
These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures.